Nearly two-thirds of energy input
is lost as waste heat. Now Northwestern University scientists have discovered a
surprising material that is the best in the world at converting waste heat to
useful electricity. This outstanding property could be exploited in solid-state
thermoelectric devices in a variety of industries, with potentially enormous
energy savings.

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An interdisciplinary team led by
inorganic chemist Mercouri G. Kanatzidis found the crystal form
of the chemical compound tin selenide conducts heat so poorly through its
lattice structure that it is the most efficient thermoelectric material known.
Unlike most thermoelectric materials, tin selenide has a simple structure, much
like that of an accordion, which provides the key to its exceptional
properties.

The efficiency of waste heat
conversion in thermoelectrics is reflected by its figure of merit, called ZT.
Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650
degrees Celsius. The material’s extremely low thermal conductivity boosts the
ZT to this high level, while still retaining good electrical conductivity.

The ZT metric represents a ratio of
electrical conductivity and thermoelectric power in the numerator (which needs
to be high) and thermal conductivity in the denominator (which needs to be
low).

Potential areas of application for
the high-temperature thermoelectric material include the automobile industry (a
significant amount of gasoline’s potential energy goes out of a vehicle’s
tailpipe), heavy manufacturing industries (such as glass and brick making,
refineries, coal- and gas-fired power plants) and places where large combustion
engines operate continuously (such as in large ships and tankers).

"A good thermoelectric material is
a business proposition -- as much commercial as it is scientific," said Vinayak P. Dravid, a senior researcher on the
team. "You don't have to convert much of the world's wasted energy into useful
energy to make a material very exciting. We need a portfolio of solutions to
the energy problem, and thermoelectric materials can play an important role."

Dravid is the Abraham Harris
Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science.

Details of tin selenide, probably
among the world’s least thermally conductive crystalline materials, are
published today (April 17) by the journal Nature.

The discovery comes less than two
years after the same research group broke the world record with another
thermoelectric material they developed in the lab with a ZT of 2.2.

"The inefficiency of current
thermoelectric materials has limited their commercial use," said Kanatzidis, Professor of Chemistry at Northwestern. "We expect a tin selenide system
implemented in thermoelectric devices to be more efficient than other systems
in converting waste heat to useful electricity."